Patterning process and resist composition

ABSTRACT

A resist composition comprising a polymer comprising recurring units having an acid labile group of cyclopentyl with tert-butyl or tert-amyl pendant is coated onto a substrate, baked, exposed to high-energy radiation, PEB and developed in an organic solvent to form a negative pattern. A fine hole pattern can be formed from the resist composition with advantages including high dissolution contrast, good size control and wide depth of focus.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2013-012870 filed in Japan on Jan. 28, 2013,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a pattern forming process comprising the stepsof exposure of resist film, deprotection reaction with the aid of acidand heat, and development in an organic solvent to form a negative tonepattern in which the unexposed region is dissolved and the exposedregion is not dissolved, and a resist composition used therein.

BACKGROUND ART

In the recent drive for higher integration and operating speeds in LSIdevices, the pattern rule is made drastically finer. Thephotolithography which is currently on widespread use in the art isapproaching the essential limit of resolution determined by thewavelength of a light source. As the light source used in thelithography for resist pattern formation, g-line (436 nm) or i-line (365nm) from a mercury lamp was widely used in 1980's. Reducing thewavelength of exposure light was believed effective as the means forfurther reducing the feature size. For the mass production process of 64MB dynamic random access memories (DRAM, processing feature size 0.25 μmor less) in 1990's and later ones, the exposure light source of i-line(365 nm) was replaced by a KrF excimer laser having a shorter wavelengthof 248 nm. However, for the fabrication of DRAM with a degree ofintegration of 256 MB and 1 GB or more requiring a finer patterningtechnology (processing feature size 0.2 μm or less), a shorterwavelength light source was required. Over a decade, photolithographyusing ArF excimer laser light (193 nm) has been under activeinvestigation. It was expected at the initial that the ArF lithographywould be applied to the fabrication of 180-nm node devices. However, theKrF excimer lithography survived to the mass-scale fabrication of 130-nmnode devices. So, the full application of ArF lithography started fromthe 90-nm node. The ArF lithography combined with a lens having anincreased numerical aperture (NA) of 0.9 is considered to comply with65-nm node devices. For the next 45-nm node devices which required anadvancement to reduce the wavelength of exposure light, the F₂lithography of 157 nm wavelength became a candidate. However, for thereasons that the projection lens uses a large amount of expensive CaF₂single crystal, the scanner thus becomes expensive, hard pellicles areintroduced due to the extremely low durability of soft pellicles, theoptical system must be accordingly altered, and the etch resistance ofresist is low; the development of F₂ lithography was stopped andinstead, the ArF immersion lithography was introduced.

In the ArF immersion lithography, the space between the projection lensand the wafer is filled with water having a refractive index of 1.44.The partial fill system is compliant with high-speed scanning and whencombined with a lens having a NA of 1.3, enables mass production of45-nm node devices.

One candidate for the 32-nm node lithography is lithography usingextreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUVlithography has many accumulative problems to be overcome, includingincreased laser output, increased sensitivity, increased resolution andminimized edge roughness (LER, LWR) of resist film, defect-free MoSilaminate mask, reduced aberration of reflection mirror, and the like.

Another candidate for the 32-nm node lithography is high refractiveindex liquid immersion lithography. The development of this technologywas stopped because LUAG, a high refractive index lens candidate had alow transmittance and the refractive index of liquid did not reach thegoal of 1.8.

The process that now draws attention under the above-discussedcircumstances is a double patterning process involving a first set ofexposure and development to form a first pattern and a second set ofexposure and development to form a pattern between the first patternfeatures. A number of double patterning processes are proposed. Oneexemplary process involves a first set of exposure and development toform a photoresist pattern having lines and spaces at intervals of 1:3,processing the underlying layer of hard mask by dry etching, applyinganother layer of hard mask thereon, a second set of exposure anddevelopment of a photoresist film to form a line pattern in the spacesof the first exposure, and processing the hard mask by dry etching,thereby forming a line-and-space pattern at a half pitch of the firstpattern. An alternative process involves a first set of exposure anddevelopment to form a photoresist pattern having spaces and lines atintervals of 1:3, processing the underlying layer of hard mask by dryetching, applying a photoresist layer thereon, a second set of exposureand development to form a second space pattern on the remaining hardmask portion, and processing the hard mask by dry etching. In eitherprocess, the hard mask is processed by two dry etchings.

As compared with the line pattern, the hole pattern is difficult toreduce the feature size. In order for the prior art method to form fineholes, an attempt is made to form fine holes by under-exposure of apositive resist film combined with a hole pattern mask. This, however,results in the exposure margin being extremely narrowed. It is thenproposed to form holes of greater size, followed by thermal flow orRELACS® method to shrink the holes as developed. However, there is aproblem that control accuracy becomes lower as the pattern size afterdevelopment and the size after shrinkage differ greater and the quantityof shrinkage is greater. With the hole shrinking method, the hole sizecan be shrunk, but the pitch cannot be narrowed.

It is then proposed in Non-Patent Document 1 that a pattern ofX-direction lines is formed in a positive resist film using dipoleillumination, the resist pattern is cured, another resist material iscoated thereon, and a pattern of Y-direction lines is formed in theother resist film using dipole illumination, leaving a grid linepattern, spaces of which provide a hole pattern. Although a hole patterncan be formed at a wide margin by combining X and Y lines and usingdipole illumination featuring a high contrast, it is difficult to etchvertically staged line patterns at a high dimensional accuracy. It isproposed in Non-Patent Document 2 to form a hole pattern by exposure ofa negative resist film through a Levenson phase shift mask ofX-direction lines combined with a Levenson phase shift mask ofY-direction lines. However, the crosslinking negative resist film hasthe drawback that the resolving power is low as compared with thepositive resist film, because the maximum resolution of ultrafine holesis determined by the bridge margin.

A hole pattern resulting from a combination of two exposures of X- andY-direction lines and subsequent image reversal into a negative patterncan be formed using a high-contrast line pattern of light. Thus holeshaving a narrow pitch and fine size can be opened as compared with theprior art.

Non-Patent Document 3 reports three methods for forming hole patternsvia image reversal. The three methods are: method (1) involvingsubjecting a positive resist composition to two double-dipole exposuresof X and Y lines to form a dot pattern, depositing a SiO₂ film thereonby LPCVD, and effecting O₂—RIE for reversal of dots into holes; method(2) involving forming a dot pattern by the same steps as in (1), butusing a resist composition designed to turn alkali-soluble andsolvent-insoluble upon heating, coating a phenol-base overcoat filmthereon, effecting alkaline development for image reversal to form ahole pattern; and method (3) involving double dipole exposure of apositive resist composition and organic solvent development for imagereversal to form holes.

The organic solvent development to form a negative pattern is atraditional technique. A resist composition comprising cyclized rubberis developed using an alkene such as xylene as the developer. An earlychemically amplified resist composition comprisingpoly(tert-butoxycarbonyloxy-styrene) is developed with anisole as thedeveloper to form a negative pattern.

Recently a highlight is put on the organic solvent development again. Itwould be desirable if a very fine hole pattern, which is not achievablewith the positive tone, is resolvable through negative tone exposure. Tothis end, a positive resist composition featuring a high resolution issubjected to organic solvent development to form a negative pattern. Anattempt to double a resolution by combining two developments, alkalinedevelopment and organic solvent development is under study.

As the ArF resist composition for negative tone development with organicsolvent, positive ArF resist compositions of the prior art design may beused. Such pattern forming processes are described in Patent Documents 1to 3. These patent documents disclose resist compositions for organicsolvent development comprising a copolymer of hydroxyadamantanemethacrylate, a copolymer of norbornane lactone methacrylate, and acopolymer of methacrylate having acidic groups including carboxyl,sulfo, phenol and thiol groups substituted with two or more acid labilegroups, and pattern forming processes using the same.

Further, Patent Document 4 discloses a process for forming a patternthrough organic solvent development in which a protective film isapplied onto a resist film. Patent Document 5 discloses a topcoatlessprocess for forming a pattern through organic solvent development inwhich an additive is added to a resist composition so that the additivemay segregate at the resist film surface after spin coating to providethe surface with improved water repellency.

The positive development system involving deprotection reaction togenerate a carboxyl group and subsequent neutralization reaction withaqueous alkaline developer to improve a dissolution rate achieves a highdissolution contrast in that the dissolution rate differs between theunexposed and exposed regions by a factor of more than 1,000. Incontrast, the negative development system via organic solventdevelopment provides a low contrast because the dissolution rate in theunexposed region due to solvation is low, and the dissolution rate thusdiffers between the unexposed and exposed regions by a factor of lessthan 100. For the negative development system via organic solventdevelopment, it is desired to seek for a novel material which can offera high dissolution contrast.

CITATION LIST

-   Patent Document 1: JP-A 2008-281974-   Patent Document 2: JP-A 2008-281975-   Patent Document 3: JP 4554665-   Patent Document 4: JP 4590431-   Patent Document 5: JP-A 2008-309879-   Non-Patent Document 1: Proc. SPIE Vol. 5377, p. 255 (2004)-   Non-Patent Document 2: IEEE IEDM Tech. Digest 61 (1996)-   Non-Patent Document 3: Proc. SPIE Vol. 7274, p. 72740N (2009)

DISCLOSURE OF INVENTION

The organic solvent development is low in dissolution contrast, ascompared with the positive resist system adapted to be dissolved inalkaline developer when deprotection reaction takes place to produceacidic carboxyl or phenol groups. Specifically, in the case of alkalinedeveloper, the alkali dissolution rate differs more than 1,000 timesbetween unexposed and exposed regions, whereas the difference in thecase of organic solvent development is at most 100 times, and only about10 times for certain materials. No sufficient margin is available. Inthe case of aqueous alkaline development, the dissolution rate isimproved by neutralization reaction with carboxyl groups. In the case oforganic solvent development with no accompanying reaction, thedissolution rate is low because dissolution is solely due to solvation.It is necessary not only to improve the dissolution rate of theunexposed region, but also to reduce the dissolution rate of the exposedregion that is a remaining portion of resist film. If the dissolutionrate of the exposed region is high, the thickness of the remaining filmis so reduced that the underlying substrate may not be processed byetching through the pattern as developed. Further it is important toenhance the gradient or gamma (γ) at the dose corresponding todissolution/non-dissolution conversion. A low γ value is prone to forman inversely tapered profile and allows for pattern collapse in the caseof a line pattern. To obtain a perpendicular pattern, the resist musthave a dissolution contrast having a γ value as high as possible.

While prior art photoresist compositions of the alkaline aqueoussolution development type are described in Patent Documents 1 to 3, theyhave a low dissolution contrast upon organic solvent development. Itwould be desirable to have a novel material having a significantdifference in dissolution rate between the exposed and unexposed regionsand capable of achieving a high dissolution contrast (γ) upon organicsolvent development.

When an attempt is made to form a hole pattern through negativedevelopment, regions surrounding the holes receive light so that excessacid is generated therein. Since the holes are not opened if the aciddiffuses inside the holes, control of acid diffusion is also important.

An object of the invention is to provide a negative pattern-formingresist composition which has a significant dissolution contrast and ahigh sensitivity upon organic solvent development. Another object is toprovide a pattern forming process capable of forming a hole pattern viapositive/negative reversal by organic solvent development.

The inventors have found that a polymer comprising recurring unitshaving an acid labile group of tertiary ester type, specifically an acidlabile group of cyclopentyl having a tert-butyl or tert-amyl pendantgroup is effective as a base resin and that a resist compositioncomprising the polymer, when processed by exposure, PEB and organicsolvent development, is improved in dissolution contrast.

When an acid labile group of such type is applied to a positive resistfilm subject to alkaline development, the high lipophilicity of branchedalkyl pendant allows for swelling in developer and top bulging, givingrise to problems like pattern collapse, LWR degradation and bridgingbetween pattern features. In negative pattern formation via organicsolvent development, the branched alkyl pendant contributes tosignificant improvements in developer solubility and acid lability,leading to high dissolution contrast. Since the polymer is not swollenat all in organic solvent developer, the problems of pattern collapse orcrush, bridge defects, and LWR degradation are eliminated. As aconsequence, a fine hole pattern can be formed with advantages includinghigh sensitivity, dimensional uniformity and wide depth of focus.

In one aspect, the invention provides a pattern forming processcomprising the steps of:

applying a resist composition comprising a polymer and an optional acidgenerator onto a substrate, said polymer comprising recurring units (a1)having an acid labile group of formula (1a), represented by the generalformula (1):

wherein R¹ is hydrogen or methyl, and the broken line designates avalence bond,

prebaking the composition to form a resist film,

exposing a selected region of the resist film to high-energy radiation,

baking, and

developing the exposed film in an organic solvent-based developer toform a negative pattern wherein the unexposed region of resist film isdissolved away and the exposed region of resist film is not dissolved.

In one preferred embodiment, polymer further comprises recurring units(a2) represented by the general formula (2) and recurring units of atleast one type selected from recurring units (b1) to (b4) represented bythe general formula (3).

Herein R¹⁰ is hydrogen or methyl, R¹¹ is an acid labile group other thanformula (1a), and X¹ is a single bond, phenylene, naphthylene, or—C(═O)—O—R¹²— wherein R¹² is a straight, branched or cyclic C₁-C₁₀alkylene group which may contain an ether radical, ester radical,lactone ring or hydroxyl radical, or a phenylene or naphthylene group,a2 is a number in the range: 0≦a2<1.0.

Herein R¹³ and R¹⁶ each are hydrogen or methyl; R¹⁴ is a di- topentavalent, straight, branched or cyclic C₁-C₁₆ aliphatic hydrocarbongroup which may contain an ether or ester radical; R¹⁵ and R¹⁷ each arean acid labile group; R¹⁸ to R²¹ and R²² to R²⁵ are each independentlyhydrogen, cyano, a straight, branched or cyclic C₁-C₆ alkyl group,alkoxycarbonyl, or a group having an ether radical or lactone ring, atleast one of R¹⁸ to R²¹ and R²² to R²⁵ has a hydroxyl group substitutedwith an acid labile group; m is an integer of 1 to 4, n is 0 or 1, b1,b2, b3 and b4 are numbers in the range: 0≦b1<1.0, 0≦b2<1.0, 0≦b3<1.0,0≦b4<1.0, 0≦b1+b2+b3+b4<1.0, and 0<a2+b1+b2+b3+b4<1.0.

In another preferred embodiment, the polymer further comprises recurringunits derived from a monomer having an adhesive group, but not an acidlabile group, the adhesive group being selected from the groupconsisting of a hydroxyl, cyano, carbonyl, ester, ether, lactone ring,carboxyl, carboxylic anhydride, sulfonic acid ester, disulfone andcarbonate group.

In a further preferred embodiment, the polymer further comprisesrecurring units of at least one type selected from recurring units (d1),(d2) and (d3) represented by the general formula.

Herein R⁰²⁰, R⁰²⁴ and R⁰²⁸ each are hydrogen or methyl, R⁰²¹ is a singlebond, phenylene, —O—R⁰³³—, or —C(═O)—Y—R⁰³³—, Y is oxygen or NH, R⁰³³ isa straight, branched or cyclic C₁-C₆ alkylene group, alkenylene group orphenylene group, which may contain a carbonyl (—CO—), ester (—COO—),ether (—O—) or hydroxyl radical, R⁰²², R⁰²³, R⁰²⁵, R⁰²⁶, R⁰²⁷, R⁰²⁹, andR⁰³¹ are each independently a straight, branched or cyclic C₁-C₁₂ alkylgroup which may contain a carbonyl, ester or ether radical, or a C₆-C₁₂aryl, C₇-C₂₀ aralkyl, or thiophenyl group, Z¹ is a single bond,methylene, ethylene, phenylene, fluorophenylene, —O—R⁰³²—, or—C(═O)—Z²—R⁰³²—, Z² is oxygen or NH, R⁰³² is a straight, branched orcyclic C₁-C₆ alkylene group, alkenylene group or phenylene group, whichmay contain a carbonyl, ester, ether or hydroxyl radical, M is anon-nucleophilic counter ion, d1, d2 and d3 are numbers in the range:0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3, and 0≦d1+d2+d3≦0.3.

In one preferred embodiment, the resist composition comprises componentZ which is an onium salt of sulfonic acid or carboxylic acid representedby the general formula (Z1) or (Z2):

R¹¹⁶—SO₃ ⁻M₂ ⁺  (Z1)

R¹¹⁷—COO⁻M₂ ⁺  (Z2)

wherein R¹¹⁶ is a straight, branched or cyclic, monovalent hydrocarbongroup of 1 to 35 carbon atoms which may contain oxygen and in which atleast one carbon-bonded hydrogen may be substituted by fluorine, withthe proviso that fluorine is not bonded to the carbon atom atalpha-position of sulfonic acid, R¹¹⁷ is a straight, branched or cyclic,monovalent hydrocarbon group of 1 to 35 carbon atoms which may containoxygen and in which at least one carbon-bonded hydrogen may besubstituted by fluorine, with the proviso that fluorine is not bonded tothe carbon atom at alpha-position of carboxylic acid, and M₂ ⁺ is acounter cation having a substituent, which is sulfonium, iodonium orammonium cation.

More preferably, component Z is an onium salt of sulfonic acid orcarboxylic acid represented by the general formula (Z3) or (Z4):

wherein R¹¹⁸, R¹²⁰ and R¹²¹ each are hydrogen or trifluoromethyl, R¹¹⁹is hydrogen, hydroxyl, a substituted or unsubstituted, straight,branched or cyclic C₁-C₂₀ alkyl group or a substituted or unsubstitutedC₆-C₃₀ aryl group, p is an integer of 1 to 3, and M₂ ⁺ is a countercation having a substituent, which is sulfonium, iodonium or ammoniumcation.

Preferably, the developer comprises at least one organic solventselected from among 2-octanone, 2-nonanone, 2-heptanone, 3-heptanone,4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone,methylcyclohexanone, acetophenone, methyl-acetophenone, propyl acetate,butyl acetate, isobutyl acetate, amyl acetate, isoamyl acetate, butenylacetate, propyl formate, butyl formate, isobutyl formate, amyl formate,isoamyl formate, methyl valerate, methyl pentenoate, methyl crotonate,ethyl crotonate, methyl propionate, ethyl propionate, ethyl3-ethoxypropionate, methyl lactate, ethyl lactate, propyl lactate, butyllactate, isobutyl lactate, amyl lactate, isoamyl lactate, methyl2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methyl benzoate, ethylbenzoate, phenyl acetate, benzyl acetate, methyl phenylacetate, benzylformate, phenylethyl formate, methyl 3-phenylpropionate, benzylpropionate, ethyl phenylacetate, and 2-phenylethyl acetate.

Typically, the step of exposing the resist film to high-energy radiationincludes KrF excimer laser lithography of wavelength 248 nm, ArF excimerlaser lithography of wavelength 193 nm, EUV lithography of wavelength13.5 nm or EB lithography.

Another embodiment is a pattern forming process comprising the steps ofapplying a resist composition comprising a polymer comprising recurringunits (a1) having an acid labile group of formula (1a), represented byformula (1), an optional acid generator, and an organic solvent, onto asubstrate, prebaking the composition to form a resist film, forming aprotective film on the resist film, exposing a selected region of theresist film to high-energy radiation, baking, and applying an organicsolvent-based developer to dissolve away the protective film and theunexposed region of resist film for forming a negative pattern whereinthe exposed region of resist film is not dissolved.

In another aspect, the invention provides a negative pattern-formingresist composition comprising a polymer comprising recurring units (a1)having an acid labile group of formula (1a), represented by formula (1),an optional acid generator, and an organic solvent. The polymer isdissolvable in a developer selected from among 2-octanone, 2-nonanone,2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone,diisobutyl ketone, methylcyclohexanone, acetophenone,methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate,amyl acetate, isoamyl acetate, butenyl acetate, propyl formate, butylformate, isobutyl formate, amyl formate, isoamyl formate, methylvalerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methylpropionate, ethyl propionate, ethyl 3-ethoxypropionate, methyl lactate,ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, amyllactate, isoamyl lactate, methyl 2-hydroxyisobutyrate, ethyl2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate,benzyl acetate, methyl phenylacetate, benzyl formate, phenylethylformate, methyl 3-phenylpropionate, benzyl propionate, ethylphenylacetate, and 2-phenylethyl acetate.

Preferably, the polymer further comprises recurring units (a2)represented by the general formula (2) and recurring units of at leastone type selected from recurring units (b1) to (b4) represented by thegeneral formula (3), as defined above.

Also preferably, the polymer further comprises recurring units derivedfrom a monomer having an adhesive group, but not an acid labile group,the adhesive group being selected from among a hydroxyl, cyano,carbonyl, ester, ether, lactone ring, carboxyl, carboxylic anhydride,sulfonic acid ester, disulfone and carbonate group.

Also preferably, the polymer further comprises recurring units of atleast one type selected from recurring units (d1), (d2) and (d3)represented by the above formula.

The resist composition may comprise component Z which is an onium saltof sulfonic acid or carboxylic acid represented by the general formula(Z1) or (Z2), as defined above. More preferably, component Z is an oniumsalt of sulfonic acid or carboxylic acid represented by the generalformula (Z3) or (Z4), as defined above.

ADVANTAGEOUS EFFECTS OF INVENTION

The resist composition comprising a polymer comprising recurring unitshaving an acid labile group of tertiary ester type, specifically an acidlabile group of cyclopentyl having a tert-butyl or tert-amyl pendantgroup as base resin exhibits a high dissolution contrast when processedby exposure, PEB and organic solvent development. A fine hole patterncan be formed with advantages including improved focal margin (DOF) andgood dimensional control.

BRIEF DESCRIPTION OF DRAWINGS

The only FIGURE, FIG. 1 is a cross-sectional view of a patterningprocess according one embodiment of the invention. FIG. 1A shows aphotoresist film disposed on a substrate, FIG. 1B shows the resist filmbeing exposed, and FIG. 1C shows the resist film being developed in anorganic solvent.

DESCRIPTION OF EMBODIMENTS

The terms “a” and “an” herein do not denote a limitation of quantity,but rather denote the presence of at least one of the referenced item.“Optional” or “optionally” means that the subsequently described eventor circumstances may or may not occur, and that description includesinstances where the event or circumstance occurs and instances where itdoes not. As used herein, the notation (C_(n)-C_(m)) means a groupcontaining from n to m carbon atoms per group. As used herein, the term“film” is used interchangeably with “coating” or “layer.”

The abbreviations and acronyms have the following meaning.

Mw: weight average molecular weight

Mn: number average molecular weight

Mw/Mn: molecular weight distribution or dispersity

GPC: gel permeation chromatography

PEB: post-exposure bake

PAG: photoacid generator

DOF: depth of focus

Briefly stated, the invention pertains to a resist compositioncomprising a polymer comprising recurring units having an acid labilegroup of tertiary ester type which is an acid labile group ofcyclopentyl having a tert-butyl or tert-amyl pendant group, an optionalacid generator, and an organic solvent; and a pattern forming processcomprising the steps of applying the resist composition onto asubstrate, prebaking to remove the unnecessary solvent and form a resistfilm, exposing a selected region of the resist film to high-energyradiation, PEB, and developing the exposed film in an organicsolvent-based developer to form a negative pattern.

For the purpose of enhancing the dissolution rate of the unexposedregion during organic solvent development, it is effective to introducean acid labile group having alkyl group of branched structure. Solvationtakes place in the branched portion, accelerating the dissolution rate.On the other hand, an acid labile group having cyclic structure iseffective for suppressing acid diffusion.

When a tertiary ester type acid labile group composed of cyclopentylwith tert-butyl or tert-amyl pendant is incorporated, the solventsolubility is improved over the tertiary ester type acid labile groupcomposed of cyclopentyl or adamantyl free of tert-butyl or tert-amylpendant. On the other hand, the cyclic structure is effective forsuppressing acid diffusion. A negative pattern featuring low diffusionand a high contrast can be formed via organic solvent development.

The tertiary ester type acid labile group is an acid labile group ofcyclopentyl with tert-butyl or tert-amyl pendant, represented by thegeneral formula (1a). The recurring unit having a carboxyl group whosehydrogen is substituted by this tertiary ester type acid labile group isrepresented by the general formula (1).

Herein R¹ is hydrogen or methyl, and the broken line designates avalence bond.

Examples of the recurring unit (a1) of formula (1) are shown below.

In addition to the recurring units (a1), the polymer may havecopolymerized therein recurring units (a2) having an acid labile groupother than formula (1) substituted thereon, as represented by thegeneral formula (2).

Herein, R¹⁰ is hydrogen or methyl, R¹¹ is an acid labile group otherthan formula (1), and X¹ is a single bond, phenylene, naphthylene, or—C(═O)—O—R¹²— wherein R¹² is a straight, branched or cyclic C₁-C₁₀alkylene group which may contain an ether radical, ester radical,lactone ring or hydroxyl radical, or a phenylene or naphthylene group,a2 is a number in the range: 0≦a2<1.0. The acid labile group R¹¹ will bedescribed later.

In addition to the recurring units (a1) of formula (1) and recurringunits (a2) of formula (2), the polymer may have further copolymerizedtherein recurring units having a hydroxyl group substituted with an acidlabile group. Suitable recurring units having an acid labilegroup-substituted hydroxyl group include those units having the generalformulae (b1) to (b4).

Herein R¹³ and R¹⁶ are each independently hydrogen or methyl. R¹⁴ is adi- to pentavalent, straight, branched or cyclic C₁-C₁₆ aliphatichydrocarbon group which may contain an ether or ester radical. R¹⁵ andR¹⁷ each are an acid labile group. R¹⁸ to R²¹ and R²² to R²⁵ are eachindependently hydrogen, cyano, a straight, branched or cyclic C₁-C₆alkyl group, alkoxycarbonyl, or a group having an ether radical orlactone ring, at least one of R¹⁸ to R²¹ and R²² to R²⁵ has a hydroxylgroup substituted with an acid labile group. The subscript m is aninteger of 1 to 4, and n is 0 or 1, b1, b2, b3 and b4 are numbers in therange: 0≦b1<1.0, 0≦b2<1.0, 0≦b3<1.0, 0≦b4<1.0, and 0≦b1+b2+b3+b4<1.0.

Examples of the monomers from which recurring units (b1) and (b2) arederived are shown below wherein R¹³ and R¹⁶ each are hydrogen or methyl,R¹⁵ and R¹⁷ each are an acid labile group.

Examples of the monomers from which recurring units (b3) and (b4) arederived are shown below wherein R is an acid labile group.

The acid labile group R¹¹ (other than formula (1)) substituting on thecarboxyl group in formula (2) and the acid labile groups R¹⁵, R¹⁷, anyone of R¹⁸ to R²¹, and any one of R²² to R²⁵ substituting on thehydroxyl group in formula (3) may be selected from a variety of suchgroups while they may be the same or different. Suitable acid labilegroups include groups of the formula (AL-10), acetal groups of theformula (AL-11), tertiary alkyl groups of the formula (AL-12), andC₄-C₂₀ oxoalkyl groups, but are not limited thereto.

In formulae (AL-10) and (AL-11), R⁵¹ and R⁵⁴ each are a monovalenthydrocarbon group, typically straight, branched or cyclic alkyl group,of 1 to 40 carbon atoms, more specifically 1 to 20 carbon atoms, whichmay contain a heteroatom such as oxygen, sulfur, nitrogen or fluorine.R⁵² and R⁵³ each are hydrogen or a monovalent hydrocarbon group,typically straight, branched or cyclic alkyl group, of 1 to 20 carbonatoms which may contain a heteroatom such as oxygen, sulfur, nitrogen orfluorine. The subscript “a5” is an integer of 0 to 10, and especially 1to 5. Alternatively, a pair of R⁵² and R⁵³, R⁵² and R⁵⁴, or R⁵³ and R⁵⁴may bond together to form a ring, specifically aliphatic ring, with thecarbon atom or the carbon and oxygen atoms to which they are attached,the ring having 3 to 20 carbon atoms, especially 4 to 16 carbon atoms.

In formula (AL-12), R⁵⁵, R⁵⁶ and R⁵⁷ each are a monovalent hydrocarbongroup, typically straight, branched or cyclic alkyl group, of 1 to 20carbon atoms which may contain a heteroatom such as oxygen, sulfur,nitrogen or fluorine. Alternatively, a pair of R⁵⁵ and R⁵⁶, R⁵⁵ and R⁵⁷,or R⁵⁶ and R⁵⁷ may bond together to form a ring, specifically aliphaticring, with the carbon atom to which they are attached, the ring having 3to 20 carbon atoms, especially 4 to 16 carbon atoms.

Illustrative examples of the acid labile group of formula (AL-10)include tert-butoxycarbonyl, tert-butoxycarbonylmethyl,tert-amyloxycarbonyl, tert-amyloxycarbonylmethyl,1-ethoxyethoxycarbonylmethyl, 2-tetrahydropyranyloxycarbonylmethyl and2-tetrahydrofuranyloxycarbonylmethyl as well as substituent groups ofthe following formulae (AL-10)-1 to (AL-10)-10.

In formulae (AL-10)-1 to (AL-10)-10, R⁵⁸ is each independently astraight, branched or cyclic C₁-C₈ alkyl group, C₆-C₂₀ aryl group orC₇-C₂₀ aralkyl group; R⁵⁹ is hydrogen or a straight, branched or cyclicC₁-C₂₀ alkyl group; R⁶⁰ is a C₆-C₂₀ aryl group or C₇-C₂₀ aralkyl group;and a5 is an integer of 0 to 10, especially 1 to 5. It is excluded fromformula (AL-10)-1 that R⁵⁸ is tert-butyl or tert-amyl.

Illustrative examples of the acetal group of formula (AL-11) includethose of the following formulae (AL-11)-1 to (AL-11)-112.

Other examples of acid labile groups include those of the followingformula (AL-11a) or (AL-11b) while the polymer may be crosslinked withinthe molecule or between molecules with these acid labile groups.

Herein R⁶¹ and R⁶² each are hydrogen or a straight, branched or cyclicC₁-C₈ alkyl group, or R⁶¹ and R⁶² may bond together to form a ring withthe carbon atom to which they are attached, and R⁶¹ and R⁶² stand for astraight or branched C₁-C₈ alkylene group when they form a ring. R⁶³ isa straight, branched or cyclic C₁-C₁₀ alkylene group. Each of b5 and d5is 0 or an integer of 1 to 10, preferably 0 or an integer of 1 to 5, andc5 is an integer of 1 to 7. “A” is a (c5+1)-valent aliphatic oralicyclic saturated hydrocarbon group, aromatic hydrocarbon group orheterocyclic group having 1 to 50 carbon atoms, which may be separatedby a heteroatom such as oxygen, sulfur or nitrogen or in which somehydrogen atoms attached to carbon atoms may be substituted by hydroxyl,carboxyl, carbonyl radicals or fluorine atoms. “B” is —CO—O—, —NHCO—O—or —NHCONH—.

Preferably, “A” is selected from divalent to tetravalent, straight,branched or cyclic C₁-C₂₀ alkylene, alkanetriyl and alkanetetraylgroups, and C₆-C₃₀ arylene groups, which may be separated by aheteroatom such as oxygen, sulfur or nitrogen or in which some hydrogenatoms attached to carbon atoms may be substituted by hydroxyl, carboxyl,acyl radicals or halogen atoms. The subscript c5 is preferably aninteger of 1 to 3.

The crosslinking acetal groups of formulae (AL-11a) and (AL-11b) areexemplified by the following formulae (AL-11)-113 through (AL-11)-120.

Illustrative examples of the tertiary alkyl group of formula (AL-12)include tert-butyl, triethylcarbyl, 1-ethylnorbornyl,1-methylcyclohexyl, 1-ethylcyclopentyl, and tert-amyl groups as well asthose of (AL-12)-1 to (AL-12)-16.

Herein R⁶⁴ is each independently a straight, branched or cyclic C₁-C₈alkyl group, C₆-C₂₀ aryl group or C₇-C₂₀ aralkyl group, or two R⁶⁴groups may bond together to form a ring. R⁶⁵ and R⁶⁷ each are hydrogen,methyl or ethyl. R⁶⁶ is a C₆-C₂₀ aryl group or C₇-C₂₀ aralkyl group.

With acid labile groups containing R⁶⁸ representative of a di- orpoly-valent alkylene or arylene group as shown by formula (AL-12)-17,the polymer may be crosslinked within the molecule or between molecules.In formula (AL-12)-17, R⁶⁴ is as defined above, R⁶⁸ is a single bond, astraight, branched or cyclic C₁-C₂₀ alkylene group or arylene group,which may contain a heteroatom such as oxygen, sulfur or nitrogen, andb6 is an integer of 0 to 3. It is noted that formula (AL-12)-17 isapplicable to all the foregoing acid labile groups.

The groups represented by R⁶⁴, R⁶⁵, R⁶⁶ and R⁶⁷ may contain a heteroatomsuch as oxygen, nitrogen or sulfur. Such groups are exemplified by thoseof the following formulae (AL-13)-1 to (AL-13)-7.

While the polymer used as the base resin in the resist compositioncomprises essentially recurring units (a1) of formula (1), andoptionally (and preferably) recurring units (a2) of formula (2), acidlabile group-containing recurring units (b1) to (b4) of formula (3), itmay have further copolymerized therein recurring units (c) derived froma monomer having an adhesive group, but not an acid labile group.Suitable adhesive groups include hydroxyl, cyano, carbonyl, ester, ethergroups, lactone rings, carboxyl, carboxylic anhydride, sulfonic acidester, disulfone, and carbonate groups. Of these, recurring units havinglactone ring as the adhesive group are most preferred.

Examples of suitable monomers from which recurring units (c) are derivedare given below.

In a preferred embodiment, the polymer has further copolymerized thereinunits selected from sulfonium salts (d1) to (d3) represented by thegeneral formulae below.

Herein R⁰²⁰, R⁰²⁴ and R⁰²⁸ each are hydrogen or methyl. R⁰²¹ is a singlebond, phenylene, —O—R⁰³³—, or —C(═O)—Y—R⁰³³— wherein Y is oxygen or NH,and R⁰³³ is a straight, branched or cyclic C₁-C₆ alkylene group,alkenylene group or phenylene group, which may contain a carbonyl(—CO—), ester (—COO—), ether (—O—) or hydroxyl radical. R⁰²², R⁰²³,R⁰²⁵, R⁰²⁶, R⁰²⁷, R⁰²⁹, R⁰³⁰, and R⁰³¹ are each independently astraight, branched or cyclic C₁-C₁₂ alkyl group which may contain acarbonyl, ester or ether radical, or a C₆-C₁₂ aryl, C₇-C₂₀ aralkyl, orthiophenyl group. Z¹ is a single bond, methylene, ethylene, phenylene,fluorophenylene, —O—R⁰³²—, or —C(═O)—Z²—R⁰³²— where in Z is oxygen orNH, and R⁰³² is a straight, branched or cyclic C₁-C₆ alkylene group,alkenylene group or phenylene group, which may contain a carbonyl,ester, ether or hydroxyl radical. M⁻ is a non-nucleophilic counter ion,d1, d2 and d3 are numbers in the range: 0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3,and 0≦d1+d2+d3≦0.3.

Besides the recurring units described above, the polymer may havefurther copolymerized therein additional recurring units, for example,recurring units (e) having a non-leaving hydrocarbon group as describedin JP-A 2008-281980. Examples of the non-leaving hydrocarbon group otherthan those described in JP-A 2008-281980 include indene, acenaphthylene,and norbornadiene derivatives. Copolymerization of recurring units (e)having a non-leaving hydrocarbon group is effective for improving thedissolution of the polymer in organic solvent-based developer.

It is also possible to incorporate recurring units (f) having an oxiraneor oxetane ring into the polymer. Where recurring units (f) having anoxirane or oxetane ring are copolymerized in the polymer, the exposedregion of resist film will be crosslinked, leading to improvements infilm retention of the exposed region and etch resistance. Examples ofthe recurring units (f) having an oxirane or oxetane ring are givenbelow wherein R⁴¹ is hydrogen or methyl.

Appropriate molar fractions of individual recurring units (a1), (a2),(b1), (b2), (b3), (b4), (c), (d1), (d2), (d3), (e) and (f) are in therange: 0<a1≦1.0, 0≦a2≦1.0, 0≦b1≦1.0, 0≦b2<1.0, 0≦b3<1.0, 0≦b4<1.0,0≦a2+b1+b2+b3+b4<1.0, 0<c<1.0, 0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3,0≦d1+d2+d3≦0.3, 0≦e≦0.4, and 0≦f≦0.6;

more preferably 0.1≦a1≦0.9, 0≦a2≦0.9, 0.1≦a1+a2≦0.9, 0≦b1≦0.9, 0≦b2≦0.9,0≦b3≦0.9, 0≦b4≦0.9, 0.1≦a1+a2+b1+b2+b3+b4≦0.9, 0.1≦c≦0.9, 0≦d1≦0.2,0≦d2≦0.2, 0≦d3≦0.2, 0≦d1+d2+d3≦0.2, 0≦e≦0.3, and 0≦f≦0.5,provided that a1+a2+b1+b2+b3+b4+c+d1+d2+d3+e+f=1.

The polymer serving as the base resin in the resist composition used inthe pattern forming process of the invention should preferably have aweight average molecular weight (Mw) in the range of 1,000 to 500,000,and more preferably 2,000 to 15,000, as measured by GPC versuspolystyrene standards using tetrahydrofuran solvent. With too low a Mw,a film thickness loss is likely to occur upon organic solventdevelopment. A polymer with too high a Mw may lose solubility in organicsolvent and have a likelihood of footing after pattern formation.

If a polymer has a wide molecular weight distribution or dispersity(Mw/Mn), which indicates the presence of lower and higher molecularweight polymer fractions, there is a possibility that followingexposure, foreign matter is left on the pattern or the pattern profileis exacerbated. The influences of molecular weight and dispersity becomestronger as the pattern rule becomes finer. Therefore, themulti-component copolymer should preferably have a narrow dispersity(Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.5, in order to provide aresist composition suitable for micropatterning to a small feature size.

The polymer used herein may be synthesized by any desired method, forexample, by dissolving unsaturated bond-containing monomerscorresponding to the respective units (a1), (a2), (b1), (b2), (b3),(b4), (c), (d1), (d2), (d3), (e), and (f) in an organic solvent, addinga radical initiator thereto, and effecting heat polymerization. Examplesof the organic solvent which can be used for polymerization includetoluene, benzene, tetrahydrofuran, diethyl ether and dioxane. Examplesof the polymerization initiator used herein include2,2′-azobisisobutyronitrile (AIBN),2,2′-azobis(2,4-dimethylvaleronitrile), dimethyl2,2-azobis(2-methylpropionate), benzoyl peroxide, and lauroyl peroxide.Preferably the system is heated at 50 to 100° C. for polymerization totake place. The reaction time is preferably 4 to 24 hours. The acidlabile group that has been incorporated in the monomers may be kept assuch, or the product may be protected or partially protected afterpolymerization.

It is acceptable to use a blend of two or more polymers which differ incompositional ratio, molecular weight or dispersity as well as a blendof an inventive polymer and another polymer free of an acid labilegroup-substituted carboxyl group as represented by formula (1) or ablend of an inventive polymer and a polymer comprising recurring unitshaving a conventional acid labile group-substituted hydroxyl or carboxylgroup, for example, a polymer comprising recurring units (a2) and/or(b1) to (b4).

In a further embodiment, the inventive polymer may be blended with apolymer of the conventional type wherein the exposed region is dissolvedon alkaline development such as (meth)acrylate polymer, polynorbornene,cycloolefin-maleic anhydride copolymer, or ring-opening metathesispolymerization (ROMP) polymer. Also, the inventive polymer may beblended with a (meth)acrylate polymer, polynorbornene, orcycloolefin-maleic anhydride copolymer having an acid labilegroup-substituted hydroxyl group wherein the exposed region is notdissolved by alkaline development, but a negative pattern is formed byorganic solvent development.

Resist Composition

The resist composition used in the pattern forming process of theinvention may further comprise an organic solvent, and optionally, acompound capable of generating an acid in response to high-energyradiation (known as “acid generator”), dissolution regulator, basiccompound, surfactant, acetylene alcohol, and other components.

The resist composition used herein may include an acid generator inorder for the composition to function as a chemically amplified resistcomposition. Typical of the acid generator used herein is a photoacidgenerator (PAG) capable of generating an acid in response to actiniclight or radiation. The PAG may preferably be compounded in an amount of0.5 to 30 parts and more preferably 1 to 20 parts by weight per 100parts by weight of the base resin. The PAG is any compound capable ofgenerating an acid upon exposure to high-energy radiation. Suitable PAGsinclude sulfonium salts, iodonium salts, sulfonyldiazomethane,N-sulfonyloxyimide, and oxime-O-sulfonate acid generators. The PAGs maybe used alone or in admixture of two or more. Typically acid generatorsgenerate such acids as sulfonic acids, imidic acids and methide acids.Of these, sulfonic acids which are fluorinated at α-position are mostcommonly used. In case the acid labile group is an acetal group which issusceptible to deprotection, the sulfonic acid need not necessarily befluorinated at α-position. In the embodiment wherein the base polymerhas recurring units (d1), (d2) or (d3) of acid generator copolymerizedtherein, the acid generator need not be separately added.

In a preferred embodiment, the resist composition comprises component Zwhich is an onium salt of sulfonic acid or carboxylic acid representedby the general formula (Z1) or (Z2).

R¹¹⁶—SO₃ ⁻M₂ ⁺  (Z1)

R¹¹⁷—COO⁻M₂ ⁺  (Z2)

Herein R¹¹⁶ is a straight, branched or cyclic, monovalent hydrocarbongroup of 1 to 35 carbon atoms which may contain oxygen and in which atleast one carbon-bonded hydrogen may be substituted by fluorine, withthe proviso that fluorine is not bonded to the carbon atom at α-positionof sulfonic acid. R¹¹⁷ is a straight, branched or cyclic, monovalenthydrocarbon group of 1 to 35 carbon atoms which may contain oxygen andin which at least one carbon-bonded hydrogen may be substituted byfluorine, with the proviso that fluorine is not bonded to the carbonatom at α-position of carboxylic acid. M₂ ⁺ is a counter cation having asubstituent, which is sulfonium, iodonium or ammonium cation.

More preferably, component Z is an onium salt of sulfonic acid orcarboxylic acid represented by the general formula (Z3) or (Z4).

Herein R¹¹⁸, R¹²⁰ and R¹²¹ each are hydrogen or trifluoromethyl. R¹¹⁹ ishydrogen, hydroxyl, a substituted or unsubstituted, straight, branchedor cyclic C₁-C₂₀ alkyl group or a substituted or unsubstituted C₆-C₃₀aryl group, p is an integer of 1 to 3, and M₂ ⁺ is a counter cationhaving a substituent, which is sulfonium, iodonium or ammonium cation.

Inclusion of an onium salt having formula (Z3) or (Z4) as component Zenables to form a hole pattern with the advantages of further improvedfocal margin (DOF) and dimensional control. Preferably component Z isused in an amount of 0.5 to 15 parts by weight per 100 parts by weightof the base resin. Within this range, the benefits of the invention aremore effectively achievable.

Illustrative examples of component Z are given below, but not limitedthereto.

To the resist composition, a basic compound may be added. The preferredbasic compound is a compound capable of holding down the diffusion rateof acid when the acid generated by the acid generator diffuses in theresist film. The inclusion of the basic compound holds down thediffusion rate of acid in the resist film, which leads to manyadvantages including improved resolution, minimized sensitivity changefollowing exposure, reduced substrate poisoning and environmentdependency, and improved exposure latitude and pattern profile.

Exemplary basic compounds include primary, secondary and tertiary aminecompounds, specifically amine compounds having a hydroxyl, ether, ester,lactone, cyano or sulfonic acid ester group, as described in JP-A2008-111103 (U.S. Pat. No. 7,537,880), paragraphs [0146] to [0164], andcompounds having a carbamate group, as described in JP-A 2001-166476.The basic compound is preferably used in an amount of 0 to 5 parts, morepreferably 0.1 to 5 parts by weight per 100 parts by weight of the baseresin.

Examples of the organic solvent used herein are described in JP-A2008-111103, paragraphs [0144] to [0145]. Specifically, exemplarysolvents include ketones such as cyclohexanone and methyl-2-n-amylketone; alcohols such as 3-methoxybutanol, 3-methyl-3-methoxybutanol,1-methoxy-2-propanol, and 1-ethoxy-2-propanol; ethers such as propyleneglycol monomethyl ether, ethylene glycol monomethyl ether, propyleneglycol monoethyl ether, ethylene glycol monoethyl ether, propyleneglycol dimethyl ether, and diethylene glycol dimethyl ether; esters suchas propylene glycol monomethyl ether acetate (PGMEA), propylene glycolmonoethyl ether acetate, ethyl lactate, ethyl pyruvate, butyl acetate,methyl 3-methoxypropionate, ethyl 3-ethoxypropionate, tert-butylacetate, tert-butyl propionate, and propylene glycol mono-tert-butylether acetate; and lactones such as γ-butyrolactone, and mixturesthereof. Where the acid labile group used is of acetal type, ahigh-boiling alcohol solvent may be added for accelerating deprotectionreaction of acetal, for example, diethylene glycol, propylene glycol,glycerol, 1,4-butane diol, and 1,3-butane diol.

The organic solvent is preferably used in an amount of 100 to 10,000parts, more preferably 300 to 8,000 parts by weight per 100 parts byweight of the base resin.

Exemplary surfactants are described in JP-A 2008-111103, paragraphs[0165] to [0166]. Exemplary dissolution regulators are described in JP-A2008-122932 (US 2008090172), paragraphs [0155] to [0178], and exemplaryacetylene alcohols in paragraphs [0179] to [0182].

Also a polymeric additive may be added for improving the waterrepellency on surface of a resist film as spin coated. This additive maybe used in the topcoatless immersion lithography. The additive has aspecific structure with a 1,1,1,3,3,3-hexafluoro-2-propanol residue andis described in JP-A 2007-297590 and JP-A 2008-111103. The waterrepellency improver to be added to the resist composition should besoluble in the organic solvent as the developer. The water repellencyimprover of specific structure with a 1,1,1,3,3,3-hexafluoro-2-propanolresidue is well soluble in the developer. A polymer having an aminogroup or amine salt copolymerized as recurring units may serve as thewater repellent additive and is effective for preventing evaporation ofacid during PEB and avoiding any hole pattern opening failure afterdevelopment. An appropriate amount of the water repellency improver is0.1 to 20 parts, preferably 0.5 to 10 parts by weight per 100 parts byweight of the base resin.

Notably, the amounts of the dissolution regulator, surfactant andacetylene alcohol added may be determined as appropriate for theirpurpose of addition.

Process

The pattern forming process of the invention comprises the steps ofcoating a resist composition onto a substrate, prebaking the resistcomposition to form a resist film, exposing a selected region of theresist film to high-energy radiation, PEB, and developing the exposedresist film in an organic solvent developer so that the unexposed regionof resist film is dissolved and the exposed region of resist film isleft, thereby forming a negative tone resist pattern such as a hole ortrench pattern.

FIG. 1 illustrates the pattern forming process of the invention. First,the resist composition is coated on a substrate to form a resist filmthereon. Specifically, a resist film 40 of a resist composition isformed on a processable substrate 20 disposed on a substrate 10 directlyor via an intermediate intervening layer 30 as shown in FIG. 1A. Theresist film preferably has a thickness of 10 to 1,000 nm and morepreferably 20 to 500 nm. Prior to exposure, the resist film is heated orprebaked, preferably at a temperature of 60 to 180° C., especially 70 to150° C. for a time of 10 to 300 seconds, especially 15 to 200 seconds.

The substrate 10 used herein is generally a silicon substrate. Theprocessable substrate (or target film) 20 used herein includes SiO₂,SiN, SiON, SiOC, p-Si, α-Si, TiN, WSi, BPSG, SOG, Cr, CrO, CrON, MoSi,low dielectric film, and etch stopper film. The intermediate interveninglayer 30 includes hard masks of SiO₂, SiN, SiON or p-Si, an undercoat inthe form of carbon film, a silicon-containing intermediate film, and anorganic antireflective coating.

Next comes exposure depicted at 50 in FIG. 1B. For the exposure,preference is given to high-energy radiation having a wavelength of 140to 250 nm, EUV having a wavelength of 13.5 nm, and EB, and especiallyArF excimer laser radiation of 193 nm. The exposure may be done eitherin a dry atmosphere such as air or nitrogen stream or by immersionlithography in water. The ArF immersion lithography uses deionized wateror liquids having a refractive index of at least 1 and highlytransparent to the exposure wavelength such as alkanes as the immersionsolvent. In the immersion lithography, the resist film as prebaked isexposed to light through a projection lens while water is introducedbetween the resist film and the projection lens. Since this allowslenses to be designed to a NA of 1.0 or higher, formation of finerfeature size patterns is possible. The immersion lithography isimportant for the ArF lithography to survive to the 45-nm node. In thecase of immersion lithography, deionized water rinsing (or post-soaking)may be carried out after exposure for removing water droplets left onthe resist film, or a protective film may be applied onto the resistfilm after pre-baking for preventing any leach-out from the resist filmand improving water slip on the film surface.

The resist protective film used in the immersion lithography ispreferably formed from a solution of a polymer having1,1,1,3,3,3-hexafluoro-2-propanol residues which is insoluble in water,but soluble in an alkaline developer liquid, in a solvent selected fromalcohols of at least 4 carbon atoms, ethers of 8 to 12 carbon atoms, andmixtures thereof. The protective film-forming composition used hereinmay be based on a polymer comprising recurring units derived from amonomer having a 1,1,1,3,3,3-hexafluoro-2-propanol residue. While theprotective film must dissolve in the organic solvent developer, thepolymer comprising recurring units derived from a monomer having a1,1,1,3,3,3-hexafluoro-2-propanol residue dissolves in organic solventdevelopers. In particular, protective film-forming materials having1,1,1,3,3,3-hexafluoro-2-propanol residues as described in JP-A2007-025634 and JP-A 2008-003569 readily dissolve in organic solventdevelopers.

In the protective film-forming composition, an amine compound or aminesalt or a polymer having copolymerized therein recurring unitscontaining an amine compound or amine salt may be used. This componentis effective for controlling diffusion of the acid generated in theexposed region of the photoresist film to the unexposed region forthereby preventing any hole opening failure. Useful protective filmmaterials having an amine compound added thereto are described in JP-A2008-003569, and useful protective film materials having an amino groupor amine salt copolymerized are described in JP-A 2007-316448. The aminecompound or amine salt may be selected from the compounds enumerated asthe basic compound to be added to the resist composition. An appropriateamount of the amine compound or amine salt added is 0.01 to 10 parts,preferably 0.02 to 8 parts by weight per 100 parts by weight of the baseresin.

After formation of the photoresist film, deionized water rinsing (orpost-soaking) may be carried out for extracting the acid generator andthe like from the film surface or washing away particles, or afterexposure, rinsing (or post-soaking) may be carried out for removingwater droplets left on the resist film. If the acid evaporating from theexposed region during PEB deposits on the unexposed region to deprotectthe protective group on the surface of the unexposed region, there is apossibility that the surface edges of holes after development arebridged to close the holes. Particularly in the case of negativedevelopment, regions surrounding the holes receive light so that acid isgenerated therein. There is a possibility that the holes are not openedif the acid outside the holes evaporates and deposits inside the holesduring PEB. Provision of a protective film is effective for preventingevaporation of acid and for avoiding any hole opening failure. Aprotective film having an amine compound added thereto is more effectivefor preventing acid evaporation. On the other hand, a protective film towhich an acid compound such as a carboxyl or sulfo group is added orwhich is based on a polymer having copolymerized therein monomeric unitscontaining a carboxyl or sulfo group is undesirable because of apotential hole opening failure.

The other embodiment of the invention is a process for forming a patternby applying a resist composition comprising a polymer comprisingrecurring units having formula (1), an optional acid generator, and anorganic solvent onto a substrate, baking the composition to form aresist film, forming a protective film on the resist film, exposing theresist film to high-energy radiation to define exposed and unexposedregions, PEB, and applying an organic solvent-based developer to thecoated substrate to form a negative pattern wherein the unexposed regionof resist film and the protective film are dissolved and the exposedregion of resist film is not dissolved. The protective film ispreferably formed from a composition comprising a polymer bearing a1,1,1,3,3,3-hexafluoro-2-propanol residue and an amino group or aminesalt-containing compound, or a composition comprising a polymer bearinga 1,1,1,3,3,3-hexafluoro-2-propanol residue and having amino group oramine salt-containing recurring units copolymerized, the compositionfurther comprising an alcohol solvent of at least 4 carbon atoms, anether solvent of 8 to 12 carbon atoms, or a mixture thereof.

Examples of suitable recurring units having a1,1,1,3,3,3-hexafluoro-2-propanol residue include those derived fromhydroxyl-bearing monomers selected from among the monomers listed forunits (c) on pages 49, 50 and 51. Examples of the amino group-containingcompound include the amine compounds described in JP-A 2008-111103,paragraphs [0146] to [0164] as being added to photoresist compositions.Examples of the amine salt-containing compound include salts of theforegoing amine compounds with carboxylic acids or sulfonic acids.

Suitable alcohols of at least 4 carbon atoms include 1-butyl alcohol,2-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol,2-pentanol, 3-pentanol, tert-amyl alcohol, neopentyl alcohol,2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol,cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol,3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol,2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol,3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol,4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol,cyclohexanol, and 1-octanol. Suitable ether solvents of 8 to 12 carbonatoms include di-n-butyl ether, diisobutyl ether, di-sec-butyl ether,di-n-pentyl ether, diisopentyl ether, di-sec-pentyl ether, di-tert-amylether, and di-n-hexyl ether.

Exposure is preferably performed in an exposure dose of about 1 to 200mJ/cm², more preferably about 10 to 100 mJ/cm². This is followed bybaking (PEB) on a hot plate at 60 to 150° C. for 1 to 5 minutes,preferably at 80 to 120° C. for 1 to 3 minutes.

Thereafter the exposed resist film is developed in a developerconsisting of an organic solvent for 0.1 to 3 minutes, preferably 0.5 to2 minutes by any conventional techniques such as dip, puddle and spraytechniques. In this way, the unexposed region of resist film wasdissolved away, leaving a negative resist pattern 40 on the substrate 10as shown in FIG. 1C. The developer used herein is preferably selectedfrom among ketones such as 2-octanone, 2-nonanone, 2-heptanone,3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone,methylcyclohexanone, acetophenone, and methylacetophenone, and esterssuch as propyl acetate, butyl acetate, isobutyl acetate, amyl acetate,butenyl acetate, isoamyl acetate, propyl formate, butyl formate,isobutyl formate, amyl formate, isoamyl formate, methyl valerate, methylpentenoate, methyl crotonate, ethyl crotonate, methyl propionate, ethylpropionate, ethyl 3-ethoxypropionate, methyl lactate, ethyl lactate,propyl lactate, butyl lactate, isobutyl lactate, amyl lactate, isoamyllactate, methyl 2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methylbenzoate, ethyl benzoate, phenyl acetate, benzyl acetate, methylphenylacetate, benzyl formate, phenylethyl formate, methyl3-phenylpropionate, benzyl propionate, ethyl phenylacetate, and2-phenylethyl acetate, and mixtures thereof. One or more of thesesolvents may be used as the developer. When a mixture of plural solventsis used, they may be mixed in any desired ratio. A surfactant may beadded to the developer while it may be selected from the same list ofcompounds as exemplified for the surfactant to be added to the resistcomposition.

At the end of development, the resist film is rinsed. As the rinsingliquid, a solvent which is miscible with the developer and does notdissolve the resist film is preferred. Suitable solvents includealcohols of 3 to 10 carbon atoms, ether compounds of 8 to 12 carbonatoms, alkanes, alkenes, and alkynes of 6 to 12 carbon atoms, andaromatic solvents. Specifically, suitable alkanes of 6 to 12 carbonatoms include hexane, heptane, octane, nonane, decane, undecane,dodecane, methylcyclopentane, dimethylcyclopentane, cyclohexane,methylcyclohexane, dimethylcyclohexane, cycloheptane, cyclooctane, andcyclononane. Suitable alkenes of 6 to 12 carbon atoms include hexene,heptene, octene, cyclohexene, methylcyclohexene, dimethylcyclohexene,cycloheptene, and cyclooctene. Suitable alkynes of 6 to 12 carbon atomsinclude hexyne, heptyne, and octyne. Suitable alcohols of 3 to 10 carbonatoms include n-propyl alcohol, isopropyl alcohol, 1-butyl alcohol,2-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol,2-pentanol, 3-pentanol, tert-amyl alcohol, neopentyl alcohol,2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol,cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol,3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol,2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol,3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol,4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol,cyclohexanol, and 1-octanol. Suitable ether compounds of 8 to 12 carbonatoms include di-n-butyl ether, diisobutyl ether, di-sec-butyl ether,di-n-pentyl ether, diisopentyl ether, di-sec-pentyl ether, di-tert-amylether, and di-n-hexyl ether. The solvents may be used alone or inadmixture. Besides the foregoing solvents, aromatic solvents may beused, for example, toluene, xylene, ethylbenzene, isopropylbenzene,tert-butylbenzene and mesitylene. Rinsing is effective for minimizingthe risks of resist pattern collapse and defect formation.

However, rinsing is not essential. If rinsing is omitted, the amount ofsolvent used may be reduced.

A hole pattern after reversal may be shrunk by the RELACS® process. Ahole pattern is shrunk by coating a shrink agent thereto, and bakingsuch that the shrink agent may undergo crosslinking at the resistsurface as a result of the acid catalyst diffusing from the resist layerduring bake, and the shrink agent may attach to the sidewall of the holepattern. The bake is at a temperature of 70 to 180° C., preferably 80 to170° C., for a time of 10 to 300 seconds. The extra shrink agent isstripped and the hole pattern is shrunk.

Where a hole pattern is formed by negative tone development, exposure bydouble dipole illuminations of X- and Y-direction line patterns providesthe highest contrast light. The contrast may be further increased bycombining dipole illumination with s-polarized illumination.

When a halftone phase shift mask bearing a lattice-like shifter patternis used, a pattern of holes may be formed at the intersections betweengratings of the lattice-like shifter pattern after development, asdescribed in JP-A 2011-170316, paragraph [0097] (US 20110177462). Thepreferred halftone phase shift mask bearing a lattice-like shifterpattern has a transmittance of 3 to 15%. More preferably, the phaseshift mask used is a phase shift mask including a lattice-like firstshifter having a line width equal to or less than a half pitch and asecond shifter arrayed on the first shifter and consisting of lineswhose on-wafer size is 2 to 30 nm thicker than the line width of thefirst shifter, whereby a pattern of holes is formed only where the thickshifter is arrayed. Also preferably, the phase shift mask used is aphase shift mask including a lattice-like first shifter having a linewidth equal to or less than a half pitch and a second shifter arrayed onthe first shifter and consisting of dots whose on-wafer size is 2 to 100nm thicker than the line width of the first shifter, whereby a patternof holes is formed only where the thick shifter is arrayed.

Exposure by double dipole illuminations of X- and Y-direction linescombined with polarized illumination presents a method of forming lightof the highest contrast. This method, however, has the drawback that thethroughput is substantially reduced by double exposures and maskexchange therebetween. To continuously carry out two exposures whileexchanging a mask, the exposure tool must be equipped with two maskstages although the existing exposure tool includes a single mask stage.Higher throughputs may be obtained by carrying out exposure of Xdirection lines continuously on 25 wafers in a front-opening unified pod(FOUP), exchanging the mask, and carrying out exposure continuously onthe same 25 wafers, rather than exchanging a mask on every exposure of asingle wafer. However, a problem arises that as the time duration untilthe first one of 25 wafers is exposed in the second exposure isprolonged, the environment affects the resist such that the resist afterdevelopment may change its size and shape. To block the environmentalimpact on wafers in standby until the second exposure, it is effectivethat the resist film is overlaid with a protective film.

To proceed with a single mask, it is proposed in Non-Patent Document 1to carry out two exposures by dipole illuminations in X and Y directionsusing a mask bearing a lattice-like pattern. When this method iscompared with the above method using two masks, the optical contrast issomewhat reduced, but the throughput is improved by the use of a singlemask. As described in Non-Patent Document 1, the method involves formingX-direction lines in a first photoresist film by X-direction dipoleillumination using a mask bearing a lattice-like pattern, insolubilizingthe X-direction lines by light irradiation, coating a second photoresistfilm thereon, and forming Y-direction lines by Y-direction dipoleillumination, thereby forming holes at the interstices between X- andY-direction lines. Although only a single mask is needed, this methodincludes additional steps of insolubilizing the first photoresistpattern between the two exposures, and coating and developing the secondphotoresist film. Then the wafer must be removed from the exposure stagebetween the two exposures, giving rise to the problem of an increasedalignment error. To minimize the alignment error between two exposures,two exposures must be continuously carried out without removing thewafer from the exposure stage. The addition of s-polarized illuminationto dipole illumination provides a further improved contrast and is thuspreferably employed. After two exposures for forming X- and Y-directionlines using a lattice-like mask are performed in an overlapping manner,negative tone development is performed whereupon a hole pattern isformed.

When it is desired to form a hole pattern via a single exposure using alattice-like mask, a quadru-pole illumination or cross-pole illuminationis used. The contrast may be improved by combining it with X-Y polarizedillumination or azimuthally polarized illumination of circularpolarization.

In the hole pattern forming process using the resist composition of theinvention, when two exposures are involved, these exposures are carriedout by changing the illumination and mask for the second exposure fromthose for the first exposure, whereby a fine size pattern can be formedat the highest contrast and to dimensional uniformity. The masks used inthe first and second exposures bear first and second patterns ofintersecting lines whereby a pattern of holes at intersections of linesis formed in the resist film after development. The first and secondlines are preferably at right angles although an angle of intersectionother than 90° may be employed. The first and second lines may have thesame or different size and/or pitch. If a single mask bearing firstlines in one area and second lines in a different area is used, it ispossible to perform first and second exposures continuously. In thiscase, however, the maximum area available for exposure is one half.Notably, the continuous exposures lead to a minimized alignment error.Of course, the single exposure provides a smaller alignment error thanthe two continuous exposures.

When two exposures are performed using a single mask without reducingthe exposure area, the mask pattern may be a lattice-like pattern, a dotpattern, or a combination of a dot pattern and a lattice-like pattern.The use of a lattice-like pattern contributes to the most improved lightcontrast, but has the drawback of a reduced resist sensitivity due to alowering of light intensity. On the other hand, the use of a dot patternsuffers a lowering of light contrast, but provides the merit of animproved resist sensitivity.

Where holes are arrayed in horizontal and vertical directions, theabove-described illumination and mask pattern are used. Where holes arearrayed at a different angle, for example, at an angle of 45°, a mask ofa 45° arrayed pattern is combined with dipole illumination or cross-poleillumination.

Where two exposures are performed, a first exposure by a combination ofdipole illumination with polarized illumination for enhancing thecontrast of X-direction lines is followed by a second exposure by acombination of dipole illumination with polarized illumination forenhancing the contrast of Y-direction lines. Two continuous exposureswith the X- and Y-direction contrasts emphasized through a single maskcan be performed on a currently commercially available scanner.

The method of combining X and Y polarized illuminations with cross-poleillumination using a mask bearing a lattice-like pattern can form a holepattern through a single exposure, despite a slight lowering of lightcontrast as compared with two exposures of dipole illumination. Themethod is estimated to attain a substantial improvement in throughputand avoids the problem of misalignment between two exposures. Using sucha mask and illumination, a hole pattern of the order of 40 nm can beformed at a practically acceptable cost.

On use of a mask bearing a lattice-like pattern, light is fully shieldedat intersections between gratings. A fine hole pattern may be formed byperforming exposure through a mask bearing such a pattern and organicsolvent development entailing positive/negative reversal.

On use of a mask bearing a dot pattern, although the contrast of anoptical image is low as compared with the lattice-like pattern mask, theformation of a hole pattern is possible owing to the presence of blackor light shielded spots.

It is difficult to form a fine hole pattern that holes are randomlyarrayed at varying pitch and position. The super-resolution technologyusing off-axis illumination (such as dipole or cross-pole illumination)in combination with a phase shift mask and polarization is successful inimproving the contrast of dense (or grouped) patterns, but not so thecontrast of isolated patterns.

When the super-resolution technology is applied to repeating densepatterns, the pattern density bias between dense and isolated patterns,known as proximity bias, becomes a problem. As the super-resolutiontechnology used becomes stronger, the resolution of a dense pattern ismore improved, but the resolution of an isolated pattern remainsunchanged. Then the proximity bias is exaggerated. In particular, anincrease of proximity bias in a hole pattern resulting from furtherminiaturization poses a serious problem. One common approach taken tosuppress the proximity bias is by biasing the size of a mask pattern.Since the proximity bias varies with properties of a photoresistcomposition, specifically dissolution contrast and acid diffusion, theproximity bias of a mask varies with the type of photoresistcomposition. For a particular type of photoresist composition, a maskhaving a different proximity bias must be used. This adds to the burdenof mask manufacturing. Then the pack and unpack (PAU) method is proposedin Proc. SPIE Vol. 5753, p171 (2005), which involves strongsuper-resolution illumination of a first positive resist to resolve adense hole pattern, coating the first positive resist pattern with anegative resist film material in alcohol solvent which does not dissolvethe first positive resist pattern, exposure and development of anunnecessary hole portion to close the corresponding holes, therebyforming both a dense pattern and an isolated pattern. One problem of thePAU method is misalignment between first and second exposures, as theauthors point out in the report. The hole pattern which is not closed bythe second development experiences two developments and thus undergoes asize change, which is another problem.

To form a random pitch hole pattern by organic solvent developmententailing positive/negative reversal, a mask is used in which alattice-like pattern is arrayed over the entire surface and the width ofgratings is thickened only where holes are to be formed as described inJP-A 2011-170316, paragraph [0102].

Also useful is a mask in which a lattice-like pattern is arrayed overthe entire surface and thick dots are disposed only where holes are tobe formed.

On use of a mask bearing no lattice-like pattern arrayed, holes aredifficult to form, or even if holes are formed, a variation of mask sizeis largely reflected by a variation of hole size because the opticalimage has a low contrast.

Example

Examples of the invention are given below by way of illustration and notby way of limitation. The abbreviation “pbw” is parts by weight. For allpolymers, Mw and Mn are determined by GPC versus polystyrene standardsusing tetrahydrofuran solvent.

Examples and Comparative Examples Polymers (Composition, Mw and Mw/Mn)

Polymers used in test compositions are identified in Table 1 with theirmolar fraction (mol %) of constituent recurring units, molecular weight(Mw) and dispersity (Mw/Mn). The structure of individual recurring unitsis shown in Tables 2 and 3. In Table 2, ALU-1 and ALU-2 correspond torecurring units (a1) having an acid labile group essential for thepolymer in the inventive negative resist composition. Polymer-1 toPolymer-10 fall within the scope of the invention whereas Polymer-11 toPolymer-15 are comparative polymers.

TABLE 1 Polymer Unit 1 Fraction Unit 2 Fraction Unit 3 Fraction Unit 4Fraction Unit 5 Fraction Mw Mw/Mn 1 ALU-1 50 Unit-1 20 Unit-2 20 Unit-510 9,100 1.60 2 ALU-1 50 Unit-2 40 Unit-5 10 9,000 1.75 3 ALU-1 50Unit-1 50 8,800 1.55 4 ALU-2 50 Unit-1 30 Unit-2 20 9,500 1.60 5 ALU-125 ALU-3 25 Unit-1 20 Unit-2 20 Unit-5 10 8,800 1.65 6 ALU-1 25 ALU-4 25Unit-1 20 Unit-2 20 Unit-5 10 8,700 1.65 7 ALU-1 40 ALU-5 10 Unit-1 20Unit-2 20 Unit-5 10 9,300 1.60 8 ALU-1 25 ALU-6 25 Unit-2 40 Unit-5 109,000 1.75 9 ALU-2 50 Unit-1 50 9,100 1.80 10 ALU-2 50 Unit-3 50 9,5001.70 11 ALU-4 30 ALU-7 20 Unit-2 40 Unit-5 10 7,900 1.75 12 ALU-4 30ALU-7 20 Unit-1 20 Unit-4 20 Unit-5 10 8,300 1.70 13 ALU-6 60 Unit-2 409,100 1.65 14 ALU-3 50 Unit-2 50 8,100 1.65 15 ALU-8 50 Unit-2 40 Unit-510 9,600 1.75

TABLE 2

ALU-1

ALU-2

ALU-3

ALU-4

ALU-5

ALU-6

ALU-7

ALU-8

TABLE 3

Unit-1

Unit-2

Unit-3

Unit-4

Unit-5

Preparation of Resist Composition

Resist compositions in solution form were prepared by dissolving apolymer (identified above) and components in solvents in accordance withthe formulation of Table 4 and filtering through a Teflon® filter with apore size of 0.2 μm. Comparative resist compositions were similarlyprepared in accordance with the formulation of Table 5. The structure ofphotoacid generators (PAG-1 to 4) is shown in Table 6. The structure ofonium salts (Salt-1 to 5) is shown in Table 7. The structure of basiccompounds (A-1 to 4) as quencher is shown in Table 8.

TABLE 4 Basic Polymer PAG Onium salt compound Solvent Resist (pbw) (pbw)(pbw) (pbw) (pbw) Example 1 PR-1 Polymer-1(100) PAG-1(12) A-1(2.1)PGMEA(2,700) GBL(300) 2 PR-2 Polymer-2(100) PAG-1(14) A-1(1.5)PGMEA(2,700) GBL(300) 3 PR-3 Polymer-3(100) PAG-1(8.6) Salt-1(6.7)PGMEA(2,700) GBL(300) 4 PR-4 Polymer-3(100) PAG-3(10.6) Salt-1(6.7)PGMEA(2,700) GBL(300) 5 PR-5 Polymer-4(100) PAG-2(4) Salt-1(9)PGMEA(2,700) GBL(300) 6 PR-6 Polymer-5(100) PAG-1(7) Salt-1(7)PGMEA(2,700) PAG-4(2) GBL(300) 7 PR-7 Polymer-6(100) PAG-1(8.6)Salt-1(6.7) PGMEA(2,700) GBL(300) 8 PR-8 Polymer-7(100) PAG-1(6.6)Salt-1(7) PGMEA(2,700) PAG-4(1.8) GBL(300) 9 PR-9 Polymer-8(100)PAG-1(8.6) Salt-1(6.7) PGMEA(2,700) GBL(300) 10 PR-10 Polymer-2(70)PAG-1(8.6) Salt-1(6.7) PGMEA(2,700) Polymer-4(30) GBL(300) 11 PR 11Polymer-3(50) PAG-1(12) A-1(2.1) PGMEA(2,700) Polymer-5(50) GBL(300) 12PR-12 Polymer-9(50) PAG-1(8.6) Salt-1(6.7) PGMEA(2,700) Polymer-10(50)GBL(300) 13 PR-13 Polymer-1(100) PAG-2(4) Salt-3(6.7) PGMEA(2,700)GBL(300) 14 PR-14 Polymer-2(100) PAG-3(12) Salt-3(6.7) PGMEA(2,700)GBL(300) 15 PR 15 Polymer-4(100) PAG-3(8) Salt-4(6.7) PGMEA(2,700)GBL(300) 16 PR-16 Polymer-4(100) PAG-4(12) Salt-1(6.7) PGMEA(2,700)GBL(300) 17 PR-17 Polymer-6(100) PAG-1(8.6) Salt-4(6.7) PGMEA(2,700)GBL(300) 18 PR-18 Polymer-8(100) PAG-3(8.6) Salt-4(6.7) PGMEA(2,700)GBL(300) 19 PR-19 Polymer-9(100) PAG-4(8.6) Salt-1(6.7) PGMEA(2,700)GBL(300) 20 PR-20 Polymer-2(100) PAG-4(8.6) Salt-2(5.3) A-2(0.6)PGMEA(2,700) GBL(300) 21 PR-21 Polymer-4(100) PAG-1(8.6) Salt-5(6.7)PGMEA(2,700) GBL(300) 22 PR-22 Polymer-6(100) PAG-1(8.6) Salt-4(6.7)PGMEA(2,700) GBL(300) 23 PR-23 Polymer-8(100) PAG-2(8.6) Salt-1(3.5)A-4(1.9) PGMEA(2,700) 24 PR-24 Polymer-9(100) PAG-1(8.6) Salt-1(3.5)A-3(1.9) PGMEA(2,700) 25 PR-25 Polymer-6(100) PAG-3(8.6) Salt-4(6.7)PGMEA(2,700) GBL(300)

TABLE 5 Basic Polymer PAG Onium salt compound Solvent Resist (pbw) (pbw)(pbw) (pbw) (pbw) Comparative 1 PR-26 Polymer-11(100) PAG-1(12) A-1(2.1)PGMEA(2,700) Example GBL(300) 2 PR-27 Polymer-12(100) PAG-1(12)Salt-1(6.7) PGMEA(2,700) GBL(300) 3 PR-28 Polymer-13(100) PAG-1(8)A-1(2.1) PGMEA(2,700) GBL(300) 4 PR-29 Polymer-14(100) PAG-2(12)A-1(2.1) PGMEA(2,700) GBL(300) 5 PR-30 Polymer-15(100) PAG-1(8.6)Salt-1(6.7) PGMEA(2,700) GBL(300) 6 PR-31 Polymer-11(100) PAG-3(12)Salt-4(6.7) PGMEA(2,700) GBL(300) 7 PR-32 Polymer-12(100) PAG-2(12)Salt-2(6.7) PGMEA(2,700) GBL(300) 8 PR-33 Polymer-13(100) PAG-4(12)Salt-4(6.7) PGMEA(2,700) GBL(300) 9 PR-34 Polymer-14(100) PAG-4(12)Salt-4(6.7) PGMEA(2,700) GBL(300) 10 PR-35 Polymer-15(100) PAG-4(12)Salt-4(6.7) PGMEA(2,700) GBL(300)

TABLE 6

PAG-1

PAG-2

PAG-3

PAG-4

Table 7

Salt-1

Salt-2

Salt-3

Salt-4

Salt-5

TABLE 8

A-1

A-2

A-3

A-4

Me stands for methyl.

The organic solvents in Tables 4 and 5 are shown below.

PGMEA: propylene glycol monomethyl ether acetateGBL: gamma-butyrolactone

To all the resist compositions in Tables 4 and 5, 5.0 pbw ofalkali-soluble surfactant SF-1 and 0.1 pbw of surfactant A were added.

-   Surfactant SF-1:    poly(3,3,3-trifluoro-2-hydroxy-1,1-dimethyl-2-trifluoromethylpropyl    methacrylate/1,1,1-trifluoro-2-hydroxy-6-methyl-2-trifluoromethylhept-4-yl    methacrylate) (described in JP-A 2008-122932)

-   Surfactant A:    3-methyl-3-(2,2,2-trifluoroethoxymethyl)-oxetane/tetrahydrofuran/2,2-dimethyl-1,3-propane    diol copolymer (Omnova Solutions, Inc.)

Evaluation Method and Results

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 100 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A940 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Tables 4 and 5 was spin coated, then baked on a hotplate at 100° C. for 60 seconds to form a resist film of 90 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.9/0.72, cross-pole opening 35 deg., azimuthallypolarized illumination), exposure was performed in a varying dosethrough a 6% halftone phase shift mask bearing a lattice-like patternwith a pitch of 90 nm and a line width of 30 nm (on-wafer size). Afterthe exposure, the wafer was baked (PEB) at the temperature shown inTables 9 and 10 for 60 seconds and developed. Specifically, thedeveloper shown in Tables 9 and 10 was injected from a developmentnozzle while the wafer was spun at 30 rpm for 3 seconds, which wasfollowed by stationary puddle development for 27 seconds. The wafer wasrinsed with 4-methyl-2-pentanol, spin dried, and baked at 100° C. for 20seconds to evaporate off the rinse liquid, yielding a negative pattern.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM (CG-4000 by Hitachi High-Technologies Corp.),the size of 125 holes was measured, from which a size variation 36 wasdetermined. DOP was determined by forming resist patterns at theeffective sensitivity while changing the focus stepwise, and determininga focus range capable of affording a hole size of 50 nm±5%.

On organic solvent development, the resist compositions within the scopeof the invention form patterns having improved dimensional uniformityand wide DOF.

Table 9 reports the PEB temperature, developer used, and test resultsfor the inventive resist compositions in Table 4. Table 10 reports thePEB temperature, developer used, and test results for the comparativeresist compositions in Table 5.

TABLE 9 PEB Hole size temperature variation DOF Resist (° C.) Developer(nm) (nm) Ex- 1 PR-1 90 n-butyl acetate 3.5 0.25 am- 2 PR-2 100 n-butylacetate 3.9 0.25 ple 3 PR-3 70 n-butyl acetate 3.5 0.30 4 PR-4 100n-butyl acetate 3.4 0.25 5 PR-5 70 n-butyl acetate 3.7 0.35 6 PR-6 90n-butyl acetate 3.6 0.25 7 PR-7 95 n-butyl acetate 3.6 0.25 8 PR-8 85n-butyl acetate 3.7 0.25 9 PR-9 90 n-butyl acetate 3.8 0.30 10 PR-10 902-heptanone 3.7 0.25 11 PR-11 100 methyl benzoate 3.5 0.25 12 PR-12 75ethyl benzoate 4.0 0.35 13 PR-13 90 n-butyl acetate 3.8 0.25 14 PR-14100 n-butyl acetate 3.6 0.30 15 PR-15 80 n-butyl acetate 3.8 0.30 16PR-16 80 n-butyl acetate 4.0 0.25 17 PR-17 95 n-butyl acetate 3.6 0.2518 PR-18 90 methyl benzoate 3.5 0.25 19 PR-19 70 ethyl benzoate 3.4 0.3020 PR-20 100 2-heptanone 3.9 0.25 21 PR-21 80 n-butyl acetate 3.6 0.3022 PR-22 90 n-butyl acetate 4.0 0.30 23 PR-23 90 n-butyl acetate 4.00.35 24 PR-24 70 n-butyl acetate 3.7 0.30 25 PR-25 90 n-butyl acetate3.9 0.25

TABLE 10 PEB Hole size temperature variation DOF Resist (° C.) Developer(nm) (nm) Com- 1 PR-26 100 n-butyl acetate 5.0 0.15 par- 2 PR-27 100n-butyl acetate 4.7 0.15 ative 3 PR-28 95 n-butyl acetate 5.5 0.15 Ex- 4PR-29 95 n-butyl acetate 4.8 0.15 am- 5 PR-30 90 n-butyl acetate 5.30.10 ple 6 PR-31 100 n-butyl acetate 4.8 0.10 7 PR-32 100 2-heptanone5.3 0.15 8 PR-33 95 methyl benzoate 5.9 0.15 9 PR-34 95 n-butyl acetate5.0 0.20 10 PR-35 90 n-butyl acetate 5.2 0.15

As is evident from Tables 9 and 10, the negative pattern forming processinvolving the steps of applying a resist composition comprising apolymer comprising recurring units (a1) having an acid labile group andan optional acid generator onto a substrate, prebaking, exposing theresist film to high-energy radiation, PEB, and developing the exposedfilm in an organic solvent to form a negative pattern wherein theunexposed region is dissolved away and the exposed region is notdissolved is successful in forming hole patterns with a minimized sizevariation and at improved DOF.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed as the best mode contemplated forcarrying out this invention, but that the invention will include allembodiments falling within the scope of the appended claims.

Japanese Patent Application No. 2013-012870 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A pattern forming process comprising the steps of: applying a resistcomposition comprising a polymer and an optional acid generator onto asubstrate, said polymer comprising recurring units (a1) having an acidlabile group of formula (1a), represented by the general formula (1):

wherein R¹ is hydrogen or methyl, and the broken line designates avalence bond, prebaking the composition to form a resist film, exposinga selected region of the resist film to high-energy radiation, baking,and developing the exposed film in an organic solvent-based developer toform a negative pattern wherein the unexposed region of resist film isdissolved away and the exposed region of resist film is not dissolved.2. The process of claim 1 wherein said polymer further comprisesrecurring units (a2) represented by the general formula (2) andrecurring units of at least one type selected from recurring units (b1)to (b4) represented by the general formula (3):

wherein R¹⁰ is hydrogen or methyl, R¹¹ is an acid labile group otherthan formula (1a), and X¹ is a single bond, phenylene, naphthylene, or—C(═O)—O—R¹²— wherein R¹² is a straight, branched or cyclic C₁-C₁₀alkylene group which may contain an ether radical, ester radical,lactone ring or hydroxyl radical, or a phenylene or naphthylene group,a2 is a number in the range: 0≦a2<1.0,

wherein R¹³ and R¹⁶ each are hydrogen or methyl; R¹⁴ is a di- topentavalent, straight, branched or cyclic C₁-C₁₆ aliphatic hydrocarbongroup which may contain an ether or ester radical; R¹⁵ and R¹⁷ each arean acid labile group; R¹⁸ to R²¹ and R²² to R²⁵ are each independentlyhydrogen, cyano, a straight, branched or cyclic C₁-C₆ alkyl group,alkoxycarbonyl, or a group having an ether radical or lactone ring, atleast one of R¹⁸ to R²¹ and R²² to R²⁵ has a hydroxyl group substitutedwith an acid labile group; m is an integer of 1 to 4, n is 0 or 1, b1,b2, b3 and b4 are numbers in the range: 0≦b1<1.0, 0≦b2<1.0, 0≦b3<1.0,0≦b4<1.0, 0≦b1+b2+b3+b4<1.0, and 0<a2+b1+b2+b3+b4<1.0.
 3. The process ofclaim 1 wherein said polymer further comprises recurring units derivedfrom a monomer having an adhesive group, but not an acid labile group,the adhesive group being selected from the group consisting of ahydroxyl, cyano, carbonyl, ester, ether, lactone ring, carboxyl,carboxylic anhydride, sulfonic acid ester, disulfone and carbonategroup.
 4. The process of claim 1 wherein said polymer further comprisesrecurring units of at least one type selected from recurring units (d1),(d2) and (d3) represented by the general formula:

wherein R⁰²⁰, R⁰²⁴ and R⁰²⁸ each are hydrogen or methyl, R⁰²¹ is asingle bond, phenylene, —O—R⁰³³—, or —C(═O)—Y—R⁰³³—, Y is oxygen or NH,R⁰³³ is a straight, branched or cyclic C₁-C₆ alkylene group, alkenylenegroup or phenylene group, which may contain a carbonyl (—CO—), ester(—COO—), ether (—O—) or hydroxyl radical, R⁰²², R⁰²³, R⁰²⁵, R⁰²⁶, R⁰²⁷,R⁰²⁹, R⁰³⁰, and R⁰³¹ are each independently a straight, branched orcyclic C₁-C₁₂ alkyl group which may contain a carbonyl, ester or etherradical, or a C₆-C₁₂ aryl, C₇-C₂₀ aralkyl, or thiophenyl group, Z¹ is asingle bond, methylene, ethylene, phenylene, fluorophenylene, —O—R⁰³²—,or —C(═O)—Z²—R⁰³²—, Z² is oxygen or NH, R⁰³² is a straight, branched orcyclic C₁-C₆ alkylene group, alkenylene group or phenylene group, whichmay contain a carbonyl, ester, ether or hydroxyl radical, M⁻ is anon-nucleophilic counter ion, d1, d2 and d3 are numbers in the range:0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3, and 0<d1+d2+d3≦0.3.
 5. The process ofclaim 1 wherein the resist composition comprises component Z which is anonium salt of sulfonic acid or carboxylic acid represented by thegeneral formula (Z1) or (Z2):R¹¹⁶—SO₃ ⁻M₂ ⁺  (Z1)R¹¹⁷—COO⁻M₂ ⁺  (Z2) wherein R¹¹⁶ is a straight, branched or cyclic,monovalent hydrocarbon group of 1 to 35 carbon atoms which may containoxygen and in which at least one carbon-bonded hydrogen may besubstituted by fluorine, with the proviso that fluorine is not bonded tothe carbon atom at alpha-position of sulfonic acid, R¹¹⁷ is a straight,branched or cyclic, monovalent hydrocarbon group of 1 to 35 carbon atomswhich may contain oxygen and in which at least one carbon-bondedhydrogen may be substituted by fluorine, with the proviso that fluorineis not bonded to the carbon atom at alpha-position of carboxylic acid,and M₂ ⁺ is a counter cation having a substituent, which is sulfonium,iodonium or ammonium cation.
 6. The process of claim 5 wherein componentZ is an onium salt of sulfonic acid or carboxylic acid represented bythe general formula (Z3) or (Z4):

wherein R¹¹⁸, R¹²⁰ and R¹²¹ each are hydrogen or trifluoromethyl, R¹¹⁹is hydrogen, hydroxyl, a substituted or unsubstituted, straight,branched or cyclic C₁-C₂₀ alkyl group or a substituted or unsubstitutedC₆-C₃₀ aryl group, p is an integer of 1 to 3, and M₂ ⁺ is a countercation having a substituent, which is sulfonium, iodonium or ammoniumcation.
 7. The process of claim 1 wherein the developer comprises atleast one organic solvent selected from the group consisting of2-octanone, 2-nonanone, 2-heptanone, 3-heptanone, 4-heptanone,2-hexanone, 3-hexanone, diisobutyl ketone, methylcyclohexanone,acetophenone, methylacetophenone, propyl acetate, butyl acetate,isobutyl acetate, amyl acetate, isoamyl acetate, butenyl acetate, propylformate, butyl formate, isobutyl formate, amyl formate, isoamyl formate,methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate,methyl propionate, ethyl propionate, ethyl 3-ethoxypropionate, methyllactate, ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate,amyl lactate, isoamyl lactate, methyl 2-hydroxyisobutyrate, ethyl2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate,benzyl acetate, methyl phenylacetate, benzyl formate, phenylethylformate, methyl 3-phenylpropionate, benzyl propionate, ethylphenylacetate, and 2-phenylethyl acetate.
 8. The process of claim 1wherein the step of exposing the resist film to high-energy radiationincludes KrF excimer laser lithography of wavelength 248 nm, ArF excimerlaser lithography of wavelength 193 nm, EUV lithography of wavelength13.5 nm or EB lithography.
 9. A pattern forming process according toclaim 1, comprising the steps of: applying a resist compositioncomprising a polymer comprising recurring units (a1) having an acidlabile group of formula (1a), represented by formula (1), an optionalacid generator, and an organic solvent, onto a substrate, prebaking thecomposition to form a resist film, forming a protective film on theresist film, exposing a selected region of the resist film tohigh-energy radiation, baking, and applying an organic solvent-baseddeveloper to dissolve away the protective film and the unexposed regionof resist film for forming a negative pattern wherein the exposed regionof resist film is not dissolved.
 10. A negative pattern-forming resistcomposition comprising a polymer, an optional acid generator, and anorganic solvent, said polymer comprising recurring units (a1) having anacid labile group of formula (1a), represented by the general formula(1):

wherein R¹ is hydrogen or methyl, and the broken line designates avalence bond, said polymer being dissolvable in a developer selectedfrom the group consisting of 2-octanone, 2-nonanone, 2-heptanone,3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone,methylcyclohexanone, acetophenone, methylacetophenone, propyl acetate,butyl acetate, isobutyl acetate, amyl acetate, isoamyl acetate, butenylacetate, propyl formate, butyl formate, isobutyl formate, amyl formate,isoamyl formate, methyl valerate, methyl pentenoate, methyl crotonate,ethyl crotonate, methyl propionate, ethyl propionate, ethyl3-ethoxypropionate, methyl lactate, ethyl lactate, propyl lactate, butyllactate, isobutyl lactate, amyl lactate, isoamyl lactate, methyl2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methyl benzoate, ethylbenzoate, phenyl acetate, benzyl acetate, methyl phenylacetate, benzylformate, phenylethyl formate, methyl 3-phenylpropionate, benzylpropionate, ethyl phenylacetate, and 2-phenylethyl acetate.
 11. Theresist composition of claim 10 wherein said polymer further comprisesrecurring units (a2) represented by the general formula (2) andrecurring units of at least one type selected from recurring units (b1)to (b4) represented by the general formula (3):

wherein R¹⁰ is hydrogen or methyl, R¹¹ is an acid labile group otherthan formula (1a), and X¹ is a single bond, phenylene, naphthylene, or—C(═O)—O—R¹²— wherein R¹² is a straight, branched or cyclic C₁-C₁₀alkylene group which may contain an ether radical, ester radical,lactone ring or hydroxyl radical, or a phenylene or naphthylene group,a2 is a number in the range: 0≦a2<1.0,

wherein R¹³ and R¹⁶ each are hydrogen or methyl; R¹⁴ is a di- topentavalent, straight, branched or cyclic C₁-C₁₆ aliphatic hydrocarbongroup which may contain an ether or ester radical; R¹⁵ and R¹⁷ each arean acid labile group; R¹⁸ to R²¹ and R²² to R²⁵ are each independentlyhydrogen, cyano, a straight, branched or cyclic C₁-C₆ alkyl group,alkoxycarbonyl, or a group having an ether radical or lactone ring, atleast one of R¹⁸ to R²¹ and R²² to R²⁵ has a hydroxyl group substitutedwith an acid labile group; m is an integer of 1 to 4, n is 0 or 1, b1,b2, b3 and b4 are numbers in the range: 0≦b1<1.0, 0≦b2<1.0, 0≦b3<1.0,0≦b4<1.0, 0≦b1+b2+b3+b4<1.0, and 0<a2+b1+b2+b3+b4<1.0.
 12. The resistcomposition of claim 10 wherein said polymer further comprises recurringunits derived from a monomer having an adhesive group, but not an acidlabile group, the adhesive group being selected from the groupconsisting of a hydroxyl, cyano, carbonyl, ester, ether, lactone ring,carboxyl, carboxylic anhydride, sulfonic acid ester, disulfone andcarbonate group.
 13. The resist composition of claim 10 wherein saidpolymer further comprises recurring units of at least one type selectedfrom recurring units (d1), (d2) and (d3) represented by the generalformula:

wherein R⁰²⁰, R⁰²⁴ and R⁰²⁸ each are hydrogen or methyl, R⁰²¹ is asingle bond, phenylene, —O—R⁰³³—, or —C(═O)—Y—R⁰³³—, Y is oxygen or NH,R⁰³³ is a straight, branched or cyclic C₁-C₆ alkylene group, alkenylenegroup or phenylene group, which may contain a carbonyl (—CO—), ester(—COO—), ether (—O—) or hydroxyl radical, R⁰²², R⁰²³, R⁰²⁵, R⁰²⁶, R⁰²⁷,R⁰²⁹, R⁰³⁰, and R⁰³¹ are each independently a straight, branched orcyclic C₁-C₁₂ alkyl group which may contain a carbonyl, ester or etherradical, or a C₆-C₁₂ aryl, C₇-C₂₀ aralkyl, or thiophenyl group, Z¹ is asingle bond, methylene, ethylene, phenylene, fluorophenylene, —O—R⁰³²—,or —C(═O)—Z²—R⁰³²—, Z² is oxygen or NH, R⁰³² is a straight, branched orcyclic C₁-C₆ alkylene group, alkenylene group or phenylene group, whichmay contain a carbonyl, ester, ether or hydroxyl radical, M⁻ is anon-nucleophilic counter ion, d1, d2 and d3 are numbers in the range:0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3, and 0<d1+d2+d3≦0.3.
 14. The resistcomposition of claim 10 wherein the resist composition comprisescomponent Z which is an onium salt of sulfonic acid or carboxylic acidrepresented by the general formula (Z1) or (Z2):R¹¹⁶—SO₃ ⁻M₂ ⁻  (Z1)R¹¹⁷—COO⁻M₂ ⁺  (Z2) wherein R¹¹⁶ is a straight, branched or cyclic,monovalent hydrocarbon group of 1 to 35 carbon atoms which may containoxygen and in which at least one carbon-bonded hydrogen may besubstituted by fluorine, with the proviso that fluorine is not bonded tothe carbon atom at alpha-position of sulfonic acid, R¹¹⁷ is a straight,branched or cyclic, monovalent hydrocarbon group of 1 to 35 carbon atomswhich may contain oxygen and in which at least one carbon-bondedhydrogen may be substituted by fluorine, with the proviso that fluorineis not bonded to the carbon atom at alpha-position of carboxylic acid,and M₂ ⁺ is a counter cation having a substituent, which is sulfonium,iodonium or ammonium cation.
 15. The resist composition of claim 14wherein component Z is an onium salt of sulfonic acid or carboxylic acidrepresented by the general formula (Z3) or (Z4):

wherein R¹¹⁸, R¹²⁰ and R¹²¹ each are hydrogen or trifluoromethyl, R¹¹⁹is hydrogen, hydroxyl, a substituted or unsubstituted, straight,branched or cyclic C₁-C₂₀ alkyl group or a substituted or unsubstitutedC₆-C₃₀ aryl group, p is an integer of 1 to 3, and M₂ ⁺ is a countercation having a substituent, which is sulfonium, iodonium or ammoniumcation.